Removed use of GraphicsContext3D from SharedGraphicsContext3D

This is the last instance of GraphicsContext3D being passed around as the primary 3D API. It's only use now is for extension queries and image packing, both of which should be separated out into different utility classes.

Many files still include "GraphicsContext3D.h" in order to access the khronos headers without using the GraphicsContext3D class. A future patch will transition those to include the required headers directly.

BUG=326382

Review URL: https://codereview.chromium.org/140743009

git-svn-id: svn://svn.chromium.org/blink/trunk@166590 bbb929c8-8fbe-4397-9dbb-9b2b20218538